Probability Calculator

Calculate a probability from favourable and total possible outcomes. Enter your own values below to see probability and check the method behind it.

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Your inputs

Change the example values to match your situation.

Your result

Let’s work it out.

Fill in your inputs and select Calculate.
Your result and breakdown will appear here.

How this calculator works

For equally likely outcomes, probability = favourable outcomes ÷ total outcomes.

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What is Probability Calculator?

Probability Calculator helps you calculate a probability from favourable and total possible outcomes.

The main result is probability, with a breakdown of complementary probability. The formula, example and assumptions below explain how to interpret it.

How to use Probability Calculator

Use the example to get familiar with the form, then enter your own figures. Any rates in the example are illustrative. Check the field labels and units before calculating.

  1. Gather favourable outcomes and equally likely outcomes. Use values from the same situation or reporting period.
  2. Replace the example favourable outcomes with your own value, then complete the other fields. Check the displayed units.
  3. Select Calculate and read probability with its result breakdown. Change one input and calculate again to compare a second scenario. Reset restores the original example.
  • Enter favourable outcomes, at least 0. Decimals are accepted.
  • Enter equally likely outcomes, at least 0.000001. Decimals are accepted.

Probability Calculator: a worked example

With the inputs below, the result is 30 % (probability). Follow the example, then replace these illustrative values with your own.

Example inputs and output
Input or outputExample value
Favourable outcomes3
Equally likely outcomes10
Probability30 %
Complementary probability70 %

Understanding the Probability result

Read probability alongside complementary probability. These describe different parts of the same calculation.

Displayed results use a readable number of decimal places. When checking a result by hand, keep extra precision until the last step. To compare scenarios, change one input at a time and keep the others fixed.

  • Example Complementary probability: 70 %

Probability Calculator: assumptions and common mistakes

A useful estimate starts with the right inputs. Check the following assumptions and limits before applying the result to your situation.

  • Check the base value, sample definition and chosen operation. Display rounding should come after the calculation.

For wider guidance, visit NIST statistical handbook. The formula and scope of this specific tool are stated on this page.

Probability Calculator FAQs

What does Probability Calculator calculate?

Probability Calculator helps you calculate a probability from favourable and total possible outcomes. Its main output is probability in %.

What inputs do I need for Probability Calculator?

You need favourable outcomes and equally likely outcomes. Use the field labels and units to match your figures to the formula.

Which formula does Probability Calculator use?

For equally likely outcomes, probability = favourable outcomes ÷ total outcomes.

What is a worked example for Probability Calculator?

Example inputs: Favourable outcomes: 3; Equally likely outcomes: 10. The result is 30 % (probability).

How should I enter favourable outcomes?

Enter favourable outcomes, at least 0. Decimals are accepted.

How do I choose equally likely outcomes?

Enter equally likely outcomes, at least 0.000001. Decimals are accepted. The example uses 10; replace it if your circumstances differ.

How should I read probability?

Read probability alongside complementary probability. These describe different parts of the same calculation.

Why might my Probability Calculator result differ from my records?

Check the base value, sample definition and chosen operation. Display rounding should come after the calculation. Differences in entered values, units or rounding can also change the comparison.

Can I compare two scenarios in Probability Calculator?

Yes. Note your first probability, change favourable outcomes or another input, and select Calculate again. Editing a field clears the old result so it is not mistaken for a new calculation.

Is Probability Calculator free, and where are my inputs calculated?

This calculator is free to use without signing in. Its formula runs in your browser; calculator inputs are not sent to a calculation API. The inputs reset to the example values when you reset the form or start a fresh page.